As a technique utilizing a photoelectric encoder, a technique has existed where the movement distance or the like of a target object is measured by measuring the displacement of an interference fringe. In a measurement device utilizing this technique, pulses are counted that a light detection sensor outputs in response to a light-dark change occurring when the interference fringe moves, and hence, the displacement amount of the interference fringe is determined. In addition, on the basis of the displacement amount of the interference fringe and a distance per one period, the movement distance of the target object is obtained.
In addition, as the photoelectric encoder, there is a photoelectric encoder that generates two pseudo-sinusoidal waveforms (hereinafter, it is assumed that the pseudo-sinusoidal waveforms are a sine waveform and a cosine waveform) that are out of phase with each other by 90 degrees, from an electrical signal the light detection sensor outputs in response to the movement of an interference fringe. In this photoelectric encoder, from changes in the phases of the sine waveform and the cosine waveform that change in response to the movement of the interference fringe, the movement amount of the interference fringe is measured.
Incidentally, as for the interference fringe, in some cases, a fringe of a high degree is formed along with a fringe of a desired degree. The interference fringe of a high degree deteriorates the S/N ratio of a signal which the light detection sensor outputs, and deforms the sine waveform and the cosine waveform. Accordingly, the measurement accuracy of the displacement amount is deteriorated. A technique of the related art is disclosed in Japanese Laid-open Patent Publication No. 63-277926 or Japanese Laid-open Patent Publication No. 2011-75581.
It is desired that technology is to improve the measurement accuracy of a displacement without resulting in worsening of an S/N ratio.